DE69724147T2 - PRODUCTION OF FIBROUS PREFORMS COATED WITH A BORNITRIDE COATING AND COMPOSITES THAT CONTAIN THEM - Google Patents

Description

The present invention has as Aim to manufacture fiber preforms with a coating are covered with boron nitride; where the coating (which on the pre-formed fiber preforms) a new structure represents, and the manufacture of thermostructural (heat-resistant) Composite materials, their reinforcement texture through the fiber preforms is formed.

The applicant has for several years the solidification and compaction of composites (composite materials) carried out by means of gas, especially with SiC matrix, using the technique of chemical Infiltration in the vapor phase (CVI). This technique is very reliable and allows the production of materials with very good quality and of pieces with relatively complex shapes. As part of a reduction step of costs and time spans however, certain steps follow this technique through one step liquid be replaced.

According to the technique of chemical Infiltration in the vapor phase (CVI) becomes the fiber preform to be treated while a first time span in an oven between a graphite holder arranged to be coated with an interphase there; the interphase can in particular consist of a layer of pyrocarbon or boron nitride, where they have a lamellar microstructure having. The brackets mentioned, bulky and expensive, have have a limited lifespan and limit the loading capacity of the furnace. While a second period after a first deposition of the matrix the preform is solidified by gas, and it is possible to Equipment features can be seen for the continuation compression using gas (FR-A-2 714 076).

• – entweder vor der Phase der Imprägnierung mit dem Harz;
• – oder auf die verdichteten Vorformen (FR-A-2 707 287).

For the ultimate purpose of completely abolishing the graphite mounts, the applicant has developed a process for solidifying the fiber preforms with liquid. According to this method, the preforms are pre-impregnated with a phenol type resin; they are then subjected to a thermal treatment (which ensures the crosslinking and pyrolysis of the resin), being held by a metallic holder which is reusable for a large number of operations; on completion of the thermal treatment, they are retrieved, being sufficiently solidified to be compressed by CVI without the holding device. The preforms obtained in this way, compacted by a ceramic matrix, do not always have satisfactory mechanical properties to the extent that during the withdrawal of the polymer (during its pyrolysis) the residue of the latter does not form an efficient interphase. In the course of this method, in which the solidification is carried out using liquid, it has therefore proven necessary to produce an interphase using gas:

• - either before the impregnation phase with the resin;
• - or on the compacted preforms (FR-A-2 707 287).

To those with the presence of carbon To avoid related problems of oxidation, it is necessary an interphase and a solidifying matrix with one or more manufacture ceramic materials.

In this context, the applicant developed a new process that leads to a new product that is particularly interesting in the case of different types of execution. The method according to the invention allows it, on the surface to create a new coating structure of the fibers of the preform, which itself has the two functions of a certain thickness Interphase and the solidifying phase guaranteed. This film-like Coating coated also the fibers of the preform in a satisfactory and stable manner (A slight decrease in volume is observed at the end pyrolysis, and this coating does not detach from the fibers ab) and forms an interphase, which is a dissipative break (non-fragile fracture mode) of the final composite material.

The results on which the present Invention based have been obtained based on research that led in different directions were. The applicant has various composite materials, preliminary stages the solidifying phase and especially precursors of SiC tested, such as B. polyvinylhydrosilane (PVS, more precisely the PVS 200 of Flamel Technologies Gesellschaft) and Polysilastyrol (PSS, more precisely that PSS 400 from Nippon Soda Gesellschaft), used separately or mixed and has tested BN (boron nitride) precursors such as B. polyborazils and polycondensed polyborazine. With this last type of prepress were very surprising and get interesting results. A BN coating, with new structure and very efficient as interphase (suitable, to form an effective solidifying interphase) and as a solidifying one Phase was obtained. This BN coating is characteristic a granular, microporous (cell-like) structure.

Thus, the present invention has as its main object fiber preforms, the fibers of which are covered with a coating of boron nitride, which is intended to form an interphase between the fibers and a compression matrix of the preforms; the boron nitride coating produced on the preformed fiber preforms characteristically has a granular structure with micropores between the grains of the structure mentioned.

Such a microporous or cell-like structure, which consists of an arrangement of small grains or Granule results is complete novel for a fiber coating made of boron nitride. According to the state of the art gas boron nitride coatings actually in the generally a dense structure of the laminar type. In J. Am. Ceram. Soc., 74 (10), October 1991, 2482-2487, it was reported that a microporous Boron nitride coating was obtained using gas, but the structure this coating remained of the laminar, layered type. The Authors have porosity this obtained microporous structure with the porosity compared to turbostratic pyrocarbon (pyrocarbone turbostratique). Inside this structure is the stratification of basic structural units flawed, giving the structure its laminar, microporous character gives.

The new, granular microporous structure the boron nitride coating of the invention would in particular be micrographic made visible (see the attached figures of the present Description).

The inventors have e.g. B. boron nitride grains observed whose diameter has between 40 and 50 nm, and their arrangement a microporous Coating with a porosity of about 20%.

A granular cellular structure of the The type of the invention is particularly interesting, on the one hand because that it limits the spread of cracks under tension and on the other hand, by the final decoupling matrix / amplification of the Composite materials supported, in that it enables when it tears to be energy-distributing under tension.

Boron nitride, which in the usual way the according to the invention manufactured fiber preforms covered, thus shows characteristic Way a granular microporous Structure on. The pores of the structure are in the submicron range. You point therefore generally a diameter of less than one on average Micrometer on.

The boron nitride can be found on the fibers of the preforms are deposited with a more or less thick thickness have been. Generally, those made according to the ending Fiber preforming Fibers that are coated with a coating average thickness between 0.1 μm and 1.2 μm are covered.

Preferably a solid interphase has to form the microporous (BN) coating a thickness of 0.4 μm or more, in general between 0.4 and 1.2 μm. The mass of such a coating according to the invention, which a form solid interphase, therefore forms approximately 10 to 15% of the mass of the Fiber preform, the fibers of which are covered with the coating. It it is found at this point that according to the invention the occurrence a larger mass is in no way excluded; that is, the microporous boron nitride coating a thickness of over 1.2 μm can. In any case the thickness of the coating generally less than 5 microns.

The fiber preforms, their fibers with a boron nitride coating with a microporous structure with a thickness coated with ≤0.4 μm are; generally 0.1 μm ≤ e <0.4 μm - form nevertheless part of the main aim of the present Invention. They are new. this coating can be an interphase with suitable properties.

The fiber preforms, which with a Boron nitride coating with original structure - granular microporous or cell-like coating - covered are generally based on heat-resistant fibers which are selected from the carbon fibers and ceramic fibers of the carbide, nitride or oxide type, especially silicon carbide fibers.

In general, fiber preforms can by winding strands, Fibers or threads can be obtained by layering of unidirectional bearings (laid out of threads or ropes) or layers of two-dimensional layers (fabrics or felts), which may with each other by needling or three-dimensionally weaving Fibers or threads are connected ... The fiber preforms of the invention therefore exist according to various Manufacturing variations. You can all existing forms of fiber preforms from the stand subordinate to technology.

The second object of the invention relates the production of thermostructural composite materials, which starting from fiber preforms of the invention, which with their new Boron nitride coating are covered.

The thermostructural composite materials are obtained in a classic way, by compacting the preforms with a matrix; the compression is carried out by CVI. The matrix that occurs can consist of a carbon matrix or a Ceramic matrix exist.

The second object of the present invention is to produce carbon / carbon (C / C) composites which have a carbon fiber reinforcement with a granular, microporous BN-coated carbon matrix and a matrix of carbon and composites with a ceramic matrix (CMC) which have a reinforcement from heat-resistant fibers (carbon or ceramic) covered with granular, microporous BN and having a ceramic matrix; in particular those of the type C / SiC (fiber reinforcement made of carbon and matrix made of silicon carbide) and those of the type SiC / SiC or oxide / SiC (reinforcement with fibers based on silicon carbide or with oxides and matrix made of silicon carbide). Within the scope of the second object of the present invention, in particular composite materials with a ceramic matrix of this last type preferred and further in particular the SiC / SiC composite materials, the reinforcing fibers of the preform are covered with a coating of granular, microporous boron nitride (this coating advantageously having a sufficient thickness to form a solidifying interphase).

• – die Herstellung einer wasserfreien Lösung von polykondensiertem Borazin;
• – Imprägnieren unter trockener Atmosphäre, bevorzugt unter trockenem Stickstoff oder Argon, der Faser-Vorformen, welche einen geeigneten Oberflächenzustand für die wasserfreie Lösung aus polykondensiertem Borazin aufweisen;
• – Pyrolyse unter trockener Atmosphäre, bevorzugt unter trockenem Stickstoff oder Argon, der imprägnierten Faser-Vorformen.

The manufacturing process of the fiber preforms which are coated with boron nitride with the new structure. This method - method using liquid - has:

• - the preparation of an anhydrous solution of polycondensed borazine;
• Impregnation under a dry atmosphere, preferably under dry nitrogen or argon, of the fiber preforms which have a suitable surface condition for the anhydrous solution of polycondensed borazine;
• - Pyrolysis in a dry atmosphere, preferably under dry nitrogen or argon, of the impregnated fiber preforms.

The impregnation is done in the usual way carried out with a solution of polycondensed borazine for the case of the precursor of the boron nitride coating with granular, cell-like Structure. The monomeric, liquid, colorless very little viscous borazine, whose boiling point is only 55 ° C used directly to the fiber reinforcements of the ultimate goal Impregnating composite materials, because it evaporates from the beginning of the heat treatment, after pyrolysis, on (without producing the polymer precursor of BN). It must therefore be polymerized beforehand. As part of the process According to the invention, it is polymerized by condensation. The polymerization is carried out under conditions which condensed into one. Lead polymer, d. H. its structure is based on interconnected borazine rings. You come earlier in this text on this polycondensed borazine back, to a method of manufacturing the same: by chemically induced polymerization of the borazine and on the preparation of the impregnation solution, which is the precursor of new boron nitride coating according to the invention includes. It is now made clear that borazine, which is very susceptible to hydrolysis (it is by hydrolysis in boric acid converted, whereby ammonia forms), only in completely dry Handled atmosphere and that it can be its condensed polymer as well behaves. Given these facts, he knows Specialist that the steps of polymerization and impregnation with exclusion of moisture (in a dry atmosphere).

Impregnation with an anhydrous solution of polycondensed borazine is placed on the preformed preforms carried out, whose fibers have a suitable surface; that is generally degreased beforehand. The expert takes into account that suitable commercially available Fibers actually come greased to the market to facilitate weaving. The Degreasing is traditionally a thermal treatment. Such pretreatment (degreasing) is particularly described in FR-A-2 640 258. It is advantageously completed through a treatment to reduce the oxygen content on the surface of the fibers. This preparation before impregnation of the fiber preforms (the fibers that form the preforms) is one known to those skilled in the art Step.

As for the impregnation of the preforms, can be according to different Variants carried out which are adapted to their geometry and dimensions. she can in particular with a full bath or layer by layer.

The present solution with polycondensed As already mentioned, borazine is an anhydrous solution. In general, it is a correct solution, however it is not excluded to carry out the impregnation with brine. As solvent preferably tetrahydrofuran (THF) or any other ether is present, which is above a boiling point has the boiling point of tetrahydrofuran, and ethylene glycol dimethyl ether (or 1,2-dimethoxyethane), commonly known as monoglyme. As part of a preferred variant of the implementation of the method of the invention will use anhydrous tetrahydrofuran (THF) or Monoglyme as a solvent for the polycondensed borazine preferred.

The amount of solution to be provided is such that it can be held in the preform by capillary forces, which are generated by the proximity of the uniform fibers: it is determined by the free volume (released from the fibers) of the preform to be coated. The concentration of the polycondensed borazine in the solution is of course dependent on the desired thickness of the coating. This technical aspect of the present invention poses no problem for the person skilled in the art who is familiar with solidification processes of fiber preforms by means of liquid. It can be stated here that the calculation of the mass m _{PBA of} the polycondensed borazine for deposition on a phase in order to to obtain an interphase with the desired thickness e _f after pyrolysis, following the equation: m _polyborazine = R _{polymerization} n | πρ _barazin (Vi / Vf) · e _f · (D + e _f ) in which:
R _polyborazine is the polymerization _yield : R _{polymerization} = m _polyborazine / m _borazine ; n is the number of fibers per strand,
| is the total length of the strand that forms the preform,
ρ _borazine is the density of borazine,
D is the diameter of the fibers,
V _i / V _f is the ratio between the volume V _{i of} the borazine used, and the volume V _{f of} the boron nitride, which arises after the pyrolysis of the polyborazine intermediate.

The polycondensed borazine, precursor the novel of the invention Boron nitride coating, as already mentioned, by polymerization of Get borazin.

It is known that the borazine polymerization can proceed according to two different reaction mechanisms, which are shown here:

The first study of polymerization was carried out in 1961 by Laubengayer and co-workers (A. W. Laubengayer, P. Moaws and R. F. Proter, J. Amer. Chem. Soc., 1961, 83, 1337). From 1990 to 1994, Sneddon et al (P. J. Fazen, H. S. Beck, A. T. Lynch, E. E. Remsen and L. G. Sneddon, Chem Mater., 1990, 2, 96 and R. T. Paine and L. G. Sneddon, Chem. Tech., 1994, 29) Polyborazils by thermolysis for 48 hours at 70 ° C under vacuum received from Borazin. These authors have found that the Reaction according to a Mechanism expires which is identical to the first of the two reaction mechanisms shown above is; this means with a release of hydrogen. The hydrogen needs to be removed regularly be, and its dangerous Properties cannot be ignored ... The PolyBorazilen (from Polybiphenyl type), which according to the first of the two mechanisms mentioned above is obtained in the patent US-A-5 502 142 described as a ceramic precursor.

In 1995 the two intermediate stages borazanaphthalene and DiBorazin were isolated and characterized by ¹ H NMR and ¹¹ B NMR, by mass spectroscopy and by X-ray analysis (PJ Fazen, EE Remsen, JS Beck, PJ Carroll, AR Mc. Ghie, et LG Sneddon, Chem. Mater , 1995, 7, 1942).

• – wenn die Polymerisation ohne chemische Induktion durchgeführt wird (gemäß Stand der Technik), läuft sie durch den einen und den anderen der beiden oben genannten Mechanismen ab, wobei der erste, bei welchem die Zwischenstufe vom Typ Polybiphenyl ist, zu einer Freisetzung von Wasserstoff führt und sich über eine lange Dauer erstreckt;
• – wenn die Polymerisation in Gegenwart eines chemischen Induktors durchgeführt wird, läuft allein das zweite Reaktionsschema ab, welches über eine Zwischenstufe des Naphtalin-Typs (Borazanaphtalin) läuft. Der zweite Weg, welcher eine Ringöffnung in Gang setzt, wobei ein komplexes Aminoboran entsteht, führt nicht zur Freisetzung von gasförmigem Wasserstoff und ermöglicht es, die Reaktion in weniger als zwei Stunden durchzuführen.

Indeed, it is the success of the inventors that they have clearly shown that:

• If the polymerization is carried out without chemical induction (according to the prior art), it proceeds through one and the other of the two above-mentioned mechanisms, the first, in which the intermediate is of the polybiphenyl type, leads to a release of hydrogen and extends over a long period;
• - If the polymerization is carried out in the presence of a chemical inductor, the second reaction scheme runs alone, which runs via an intermediate stage of the naphthalene type (borazanaphtaline). The second way, which starts a ring opening, whereby a complex aminoborane is formed, does not lead to the release of gaseous hydrogen and enables the reaction to be carried out in less than two hours.

This second way of synthesis is completely new. It thus produces polycondensed borazines, ie borazine rings are connected to one another in the interior of their structure. This structure differs from the structure of the polybiphenyl type as stated above; it also differs from other structures in which the borazine rings are connected by bridges of the -NH type (see the polymers described in patent US-A-5 204 295 and application GB-A-2 163 761) or the -N (CH ₃ ) - type ver are bound (see the polymers described in application EP-A-0 448 432).

This second synthetic route of polyborazine is the preferred route to obtain polycondensed borazine as a precursor to the novel boron nitride coating of fiber preforms of the invention. It makes it possible to avoid the restrictions associated with the release of a gas (H ₂ ) during the course of the reaction and to considerably accelerate the polymerization.

Therefore, according to the preferred variant of the method of the invention that in solution for impregnation of the fiber preforms used, polycondensed borazine by chemical means Get induced polymerization of borazine; this induced Polymerization takes place under an inert atmosphere and in an anhydrous environment carried out in the presence of at least one inductor, which is preferably selected among the primary Mono- or diamines, the secondary Amines and ammonia.

• – ein primäres Amin der Formel R-NH₂, wobei R eine unverzweigte oder verzweigte Alkyl-Gruppe ist, welche 1 bis 12 Kohlenstoffatome aufweist und insbesondere primäre Amine der Formel: MeNH₂, iPrNH₂ und nBuNH₂; oder
• – ein Diamin der Formel H₂N-CH₂-(CH₂)_n-NH₂, wobei n = 1, 2, 3, 4 oder 5 und insbesondere das Ethylendiamin (H₂N-CH₂-CH₂-NH₂); oder
• – Ammoniak. Diese Polymerisation wird in diesem Zusammenhang vorzugsweise in Gegenwart eines Überschusses an gasförmigem Ammoniak oder nach Lösung des Borazins in flüssigem Ammoniak durchgeführt.

In the context of this preferred variant, the following are particularly preferred as chemical inductors:

• A primary amine of the formula R-NH ₂ , where R is an unbranched or branched alkyl group which has 1 to 12 carbon atoms and in particular primary amines of the formula: MeNH ₂ , iPrNH ₂ and nBuNH ₂ ; or
• A diamine of the formula H ₂ N-CH ₂ - (CH ₂ ) _n -NH ₂ , where n = 1, 2, 3, 4 or 5 and in particular the ethylenediamine (H ₂ N-CH ₂ -CH ₂ -NH ₂ ); or
• - ammonia. In this connection, this polymerization is preferably carried out in the presence of an excess of gaseous ammonia or after the borazine has been dissolved in liquid ammonia.

It is found that with the secondary amines (RR'NH, e.g. Et ₂ NH) the reaction requires a larger molar proportion of RR'NH in order to obtain identical mass yields. The polymerization is even slower with tertiary amines (e.g. diazabicycloundecene, DBU).

The induced (accelerated) polymerization of borazine, carried out in the context of the present invention, we generally carried out at a temperature between -100 ° C and room temperature (see can nevertheless at a higher Temperature, which is determined by the reflux temperature (boiling temperature) of the Reaction mixture is limited) with a suitable amount of inductor, the appropriate amount can vary within a wide range (e.g. 5 to 300 mole% with respect to borazine).

As an inducer of the polymerization very particularly preferred ethylenediamine or ammonia. In particular the presence of ammonia, which is in the liquid state, is preferred the good mixing of the reactants favors and with a strong excess accelerates the polymerization kinetics without triggering exothermic heating.

According to a particularly preferred Variant of the method of the invention is the polycondensed Borazin, which is used for impregnation the fiber preforms is used by polymerization of borazine in solution of an excess on liquid Obtain ammonia (5 to 10 moles of ammonia per mole of borazine) and will for the impregnation used in ethylene glycol dimethyl ether (monoglyme). The polyborazine is used after removing excess ammonia, possibly under Vacuum.

Above is polycondensed borazine, which was obtained by (preferably induced) polymerization of unsubstituted borazine. In the context of the present invention, it is in no way excluded to obtain polycondensed borazine as a precursor of the novel boron nitride coating by polymerizing substituted borazines, in particular N-alkyl and / or B-alkyl borazines (e.g. N-trimethyl borazine and N-triisopropyl borazine ). Alkyl is generally understood to mean a linear or branched (C 1 to C ₈ ) alkyl group. Thus, in the present text the term borazine encompasses the borazine and its substituted derivatives, and the term polyborazine also encompasses polymers based on polymerized borazine as well as based on its polymerized derivatives.

The method of the invention which is characteristic of an anhydrous solution of polycondensed Borazin for the impregnation phase is present, has a pyrolysis step of the impregnated Preform on which is not new. This pyrolysis step is preferably carried out under dry nitrogen or argon (in Generally at 1000 ° C). Indeed it can be between 700 and 1800 ° C carried out become.

The method of the invention - method using liquid - generated on the surface a new coating of fiber preforms (BN, previously polyborazine), which has weak volume shrinkage and which is perfect adheres to the fiber preforms (no fragment of the coating comes off off while the finished composite is loaded). In addition, the coating responsible for a decoupling of fibers and matrix, which leads to the desired dissipative tearing leads and a powerful can form a consolidating interphase.

The applicant could not find anything similar Achieve results with SiC coatings from PVS or PSS. This Coatings do not form efficient interphases.

• – die Herstellung von Faser-Vorformen gemäß dem ersten Ziel der vorliegenden Erfindung (d. h. mit einer granulären mikroporösen Bornitridbeschichtung bedeckten Faser-Vorformen) wobei die Herstellung bevorzugt gemäß dem zweiten Ziel der Erfindung (oben dargestellt) durchgeführt wird, wobei eine wasserfreie Lösung von polykondensiertem Borazin vorhanden ist, die vorzugsweise durch chemisch induzierte (beschleunigte) Polymerisation von erhalten wurde;
• – Verdichtung der mit der Beschichtung beschichteten Faser-Vorformen durch CVI.

According to the last object, the present invention relates to a method for producing composite materials which, as reinforcement, include the fiber pre-forms which are coated in a characteristic manner with the boron nitride coating with a granular microporous structure. This manufacturing process features:

• The manufacture of fiber preforms according to the first object of the present invention (ie fiber preforms covered with a granular microporous boron nitride coating), the manufacture preferably being carried out according to the second object of the invention (shown above), using an anhydrous solution of polycondensed borazine is present, which was preferably obtained by chemically induced (accelerated) polymerization of;
• - Compression of the fiber preforms coated with the coating by CVI.

The compression takes place according to one known procedures (CVI) instead. In the case where the fiber preforms are preferably obtained solidified (according to the preferred implementation variant their production: The boron nitride coatings produced have a sufficient thickness to form a solidifying interphase), you dispense with any bracket for their compression. The Densification is according to this advantageous variant carried out with solidified fiber preforms.

The invention claimed here through the attached three figures (micrographs) illustrated. These are cuts of thermostructural composite materials of the invention in various magnifications:

On 1 - Micrography with a magnification of 3000 - one can clearly see the fibers F, covered with the boron nitride coating (solidifying interphase I _BN ) of the invention, which are soaked in the matrix M (SiC obtained by CVI);

On 2 - Micrography with a magnification of 50,000 - you can see more precisely the zone of the Interphase I _BN with a thickness of approx. 0.5 μm. The granular, microporous character of Interphase I _BN appears on this print.

On 3 - Micrography with a magnification of 100,000 - a detail of the Interphase I _{BN is} shown. Their granular, microporous structure is clearly visible.

The following examples show to non-limiting Way each of the objects of the present invention.

Example 1:

Manufacture of polycondensed Polyborazine by induced polymerization by 10% ethylenediamine

In a 100 ml flask, which is in a glove box is placed under dry argon 4.0 g of borazine were introduced.

While moving the piston by rotation, a volume of ethylenediamine HN ₂ -CH ₂ -CN ₂ -NH _{2 is} injected, which corresponds to 10 mol% of the amount of borazine or 0.3 g: the strongly exothermic reaction triggers the immediate formation of a gel, accompanied by a polymer aerosol that fills the flask and also a partial elimination of borazine by evaporation due to its low boiling point (θ _b = 55 ° C).

The flask is then fitted with a reflux condenser and connected to a rooster which is a stream of inert gas, preferably dry argon (containing only 5 to 10 ppm water) is introduced to the reaction outside of the glove box to continue. Heating at 95 ° C for 5 min after a temperature rise of the same length of time is sufficient that Convert gel into a solid polyborazine foam, using only weak Traces of insolubility remain when dissolved in THF becomes.

The consequence of the uncontrollable Evaporation of part of the borazine is reproducible, in general final yield between 70 and 90%.

Implementation of the polycondensed borazine

Impregnation:

Strands of silicon carbide fibers from the manufacturer Nippon Carbon and the type Nicalon NLM 202 thermally degreased. For some of them are extended to thermal treatment their share of surface oxygen to reduce the education associated with degreasing of silica follows. This treatment after degreasing brings the weight percentage of oxygen from 16 to 13%.

The strands are then wound on a graphite frame so that 12 segments with a length of 85 mm are formed. After drying for approx. 20 min at a temperature of 120 ° C to eliminate absorbed water, the wrapped frame in the glove box is placed under dry conditions brought gon.

A preliminary test shows that each Segment of the strand is able to capillary 50 μl solution (solvent: THF), which is applied using an automatic pipette become.

The concentration of a first solution enables a thickness of the pyrolysate of 1.2 μm on each fiber one of the Obtain segments if you apply the calculated 50 μl there. It is 13.6%. The solution contains 3.3 g polycondensed borazine and 20.7 g anhydrous THF. she will as a stock solution designated. Each 50 μl sample taken on a strand segment is applied contains 3.7 mg polycondensed borazine: this leads to a thickness of boron nitride of 1.2 μm on every fiber. Allow successive dilutions of this stock solution pyrolysate thicknesses of 1.0 μm, 0.8 μm, 0.5 μm, 0.3 μm and 0.1 μm are obtained, with 50 μl on each segment this diluted solutions is applied.

Their characteristics are as follows:
[PBA] = 11.1% ⇒ m _PBA = 3.0 mg polyborazine per sample of 50 μl ⇒ 1.0 μm pyrolysate
[PBA] = 8.8% ⇒ m _PBA = 2.4 mg polyborazine per sample of 50 μl ⇒ 0.8 μm pyrolysate
[PBA] = 5.4% ⇒ m _PBA = 1.5 mg polyborazine per sample of 50 μl ⇒ 0.5 μm pyrolysate
[PBA] = 3.2% ⇒ m _PBA = 0.9 mg polyborazine per sample of 50 μl ⇒ 0.3 μm pyrolysate
[PBA] = 1.1% ⇒ m _PBA = 0.3 mg polyborazine per sample of 50 μl ⇒ 0.1 μm pyrolysate

Two segments of the strands will be with the same solution impregnated to verify the reproducibility of the method.

pyrolysis:

After evaporation of the solvent the frame from the glove box to the pyrolysis furnace becomes one filled with dry argon Desiccator transported.

The pyrolysis of the impregnated strands is carried out on their frame in a graphite furnace under a nitrogen atmosphere under the following conditions: increase at 0.5 ° C · min ^-1 to 750 ° C, then at 2 ° C · min ^-1 up to 1000 ° C, a one hour step at this temperature and return to ambient temperature according to the thermal inertia of the furnace.

Compression:

The strands are on their frame densified by deposits of silicon carbide in the vapor phase (CVI, classic method).

Characterization:

After retrieving the frame the composite strands by micrographing their orthogonal cuts and their tear zones characterized after bending.

The observations show that that Boron nitride makes the fibers uniform that covered the strands after Degreasing aftertreatment or not, and that the amount of existing polymer is more or less uniform. The investigation of the crack surfaces shows progressive damage with fiber extraction, typical of dissipative cracks.

Example 2

Manufacture of polycondensed Borazine by polymerization caused by an excess of ammonia (liquefied Ammonia) was induced

The reactants are introduced through a septum into a three-necked flask, which has been cooled to a temperature of about -60 ° C. and is constantly flushed with dry nitrogen, in the ratio of 8 moles of ammonia per mole of borazine. In the case of the present example, 14.0 g of borazine in 23 g of ammonia were introduced using a syringe which was loaded in the glove box. The mixture of the reactants is in the form of a viscous liquid which is mixed by briefly shaking. When the temperature rises again to 0 ° C, which takes about 2 hours after being removed from the cooling bath, the two reactants are in the liquid state. This enables their intensive mixing (which cannot be achieved if the ammonia (boiling point ≈ –33 ° C) was gaseous). The low temperature also makes it possible to absorb the heat generated by the highly exothermic nature of the reaction, which moderates the reactivity without premature precipitation of a part !! s !! of the polymer, the progressive crosslinking of which would have made the end product partially insoluble. At the end of the reaction, the polycondensed borazine was in the form of a foam that quickly solidified. Its mass was 17.3 g, ie greater than the mass of the borazine introduced, which indicates that the ammonia to form the Macromolecule obtained. Thus, the yield in terms of the monomeric borazine, the only predictable yield in such a case (uncontrolled excess), was greater than 100%, ranging from 120 to 125% from one trial to the next; this efficiency shows the good reproducibility of the process.

After the remaining ammonia extracted under vacuum over a period of time that did not exceed approx. Went out for 20 h, added monoglyme, resulting in a very stable, milky sol led.

The monoglyme was the THF (boiling point: 65 ° C) brought forward, due to its strongly polar character, which is the long-term stability of the sol received, and also because of its boiling point (boiling point: 84 ° C) what to greater flexibility in execution led, by facilitating better time control of stickiness the pre-impregnated Elements.

Use of the condensed Polymers

impregnation

• – Zwei der Tücher wurden nur thermisch entfettet und wurden imprägniert:
• – eines mit einem Sol, welches eine Masse an kondensiertem Polyborazin enthielt, um nach Pyrolyse eine Interphase mit einer Dicke von 1,2 μm zu ergeben; und
• – das andere mit einem Sol, welches eine Masse an kondensiertem Polyborazin enthielt, um eine Interphase mit einer Dicke von 0,4 μm nach Pyrolyse zu ergeben;
• – zwei der Tücher wurden durch Wärmebehandlung entfettet und anschließend einer Nachbehandlung zur Reduktion des Sauerstoffoberflächenanteils der Fasern, aus welchen sie gebildet sind, unterzogen und wurden imprägniert:
• – eines mit einem Sol, welches eine Masse an kondensiertem Polyborazin enthielt, um eine Interphase mit einer Dicke von 1,2 μm nach Pyrolyse zu ergeben; und
• – das andere mit einem Sol, welches eines Masse an kondensiertem Polyborazin enthielt, um eine Interphase mit einer Dicke von 0,4 μm nach Pyrolyse zu ergeben.

The goal was to make four stacks of six layers of two-way Nicalon NLM 202 fabric measuring 140mm x 50mm:

• - Two of the cloths were only degreased thermally and were impregnated:
• One with a sol which contained a mass of condensed polyborazine to give an interphase with a thickness of 1.2 μm after pyrolysis; and
• The other with a sol containing a mass of condensed polyborazine to give an interphase with a thickness of 0.4 μm after pyrolysis;
• - Two of the wipes were degreased by heat treatment and then subjected to a post-treatment to reduce the oxygen surface portion of the fibers from which they are formed, and were impregnated:
• One with a sol containing a mass of condensed polyborazine to give an interphase with a thickness of 1.2 μm after pyrolysis; and
• - the other with a sol containing a mass of condensed polyborazine to give an interphase with a thickness of 0.4 μm after pyrolysis.

• – Jeder Stapel wurde in ein Teflon-beschichtetes Gewebegefäß eingebracht, welches auf ca. 120°C für eine Zeitspanne von 30 min erhitzt wurde, und die Anordnung wurde in den Handschuhkasten eingebracht;
• – das Volumen der vier Imprägnierungssole wurde gleich gewählt wie das freie Volumen in jedem der Stapel, d. h. 15 ml,
• – die Masse des polykondensierten Borazins, das in jedem der beiden Sole zum Erzeugen einer Interphase von 1,2 μm enthalten ist, ist 4,2 g (d. h. [PBA]_M = 32%);
• – die Masse des polykondensierten Borazins, das in jedem der beiden Sole zum Erzeugen einer Interphase von 0,4 μm enthalten ist, ist 1,3 g (d. h. [PBA]_m = 10%).

The procedure was as follows:

• - Each stack was placed in a Teflon-coated tissue jar which was heated to approximately 120 ° C for a period of 30 minutes and the assembly was placed in the glove box;
• The volume of the four impregnation brines was chosen to be the same as the free volume in each of the stacks, ie 15 ml,
• The mass of the polycondensed borazine contained in each of the two brines to produce an interphase of 1.2 μm is 4.2 g (ie [PBA] _M = 32%);
• - The mass of the polycondensed borazine, which is contained in each of the two brines to produce an interphase of 0.4 μm, is 1.3 g (ie [PBA] _m = 10%).

The brine was applied to the appropriate Pour poured and dried so that the solvent evaporated.

pyrolysis

After evaporation of the solvent the transport to the furnace and the pyrolysis became as in Example 1 carried out.

compression

The compression became conventional carried out by CVI, until a density of approx. 2.3 was reached.

characterization

The mechanical properties of the compacted composites were tested. Usually tensile strength values were of 150 MPa and elongation values of 0.15 to 0.2%.

This medium performance is similar the one with the first generation SiC / SiC composites obtained with interphase from pyrolytic carbon (PyrC).

Claims (7)

Process for the production of fiber preforms, the reinforcing fibers of which are covered with a boron nitride coating intended to form an interphase between the fibers and a compression matrix of the preforms, characterized in that it comprises: - producing an anhydrous solution of polycondensed borazine ;

Impregnation of the fiber preforms, which have a suitable surface condition, with the anhydrous solution of polycondensed borazine under a dry, inert atmosphere, preferably under dry argon or nitrogen; - Pyrolysis of the impregnated fiber preforms in a dry, inert atmosphere, preferably under dry argon or nitrogen. A method according to claim 1, characterized in that the polycondensed borazine by chemically induced polymerization of the borazine is obtained; that the polymerization in dry Atmosphere and anhydrous environment in the presence of at least one inducing Agent selected from the primary Mono- or diamines, the secondary Amines and the ammonia is carried out. A method according to claim 2, characterized in that the inducing agent is a primary amine of the formula R-NH ₂ , wherein R is an unbranched or branched alkyl group having 1 to 12 carbon atoms, or a diamine of the formula HN ₂ -CH ₂ - (CH ₂ ) _n -NH ₂ with n = _{1, 2, 3, 4} or 5. Method according to one of claims 1 to 3, characterized in that that impregnation with the polycondensed borazine, dissolved in anhydrous tetrahydrofuran (THF) or anhydrous 1,2-dimethoxy-ethane (ethylene glycol dimethyl ether, Monoglyme) becomes. Method according to one of claims 1 to 4, characterized in that that the polycondensed borazine by polymerizing in liquid ammonia dissolved Borazine is obtained and that the impregnation with the polycondensed Borazine in anhydrous 1,2-dimethoxyethane (ethylene glycol dimethyl ether, monoglyme) carried out becomes. Method according to one of claims 1 to 5, characterized in that that impregnation on degreased fiber preforms, even to reduce the Proportion of surface oxygen post-treated fiber preforming is carried out. Process for the production of composite materials, characterized in that it has: - the production of fiber preforms according to one of the claims 1 to 6, the boron nitride coatings produced preferably have such a thickness that that by means of the coatings preforms obtained are solidified; - compression by CVI (chemical vapor infiltration) of the coated and preferably solidified fiber preforms.